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Soybean Aphids May Be on Their Way to Your Fields

Jeff Caldwell 08/04/2014 @ 1:39pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

Soybean aphid numbers are on the rise in parts of the Corn Belt, agronomists have discovered, and that means it's time to keep a close eye on fields and pull the trigger on treatment to avoid any yield penalties from the pest.

Iowa State University (ISU) Extension agronomist Erin Hodgson said on Monday that aphids have been discovered in fields in parts of her state, specifically the north-central portion. Entering the month of August -- the most critical time frame for the soybean crop -- it's time to get out and scout for the pest.

"This insect has the ability to cause significant yield loss during periods of optimal reproduction," she says. "Several notable infestations have been reported, particularly in north-central Iowa, this week. Therefore, scouting to determine population densities is strongly encouraged."

When scouting, remember 40 and 50. Forty is the number of aphids per plant on 50% of a field that warrants closer subsequent inspection in the coming days and weeks. The magic number is ultimately 250 -- that many of the bugs on an infected plant means it's time to treat.

"With the potential of many overlapping generations in a field, scout weekly from plant emergence until seed set to assess population dynamics," Hodgson says. "The economic threshold for soybean aphid is well established for the north-central region. Consider a foliar application when the average density exceeds 250 per plant. Populations should be increasing, and most of the plants have to be infested in order to justify an application. This threshold is appropriate until plants reach mid-seed set."

There's a wide range of products labeled for soybean aphid control, but there's a catch: Look for preharvest interval and make sure you're not pushing back your harvest by treating with a product with too wide a window, Hodgson advises.

"According to my efficacy evaluations, most products labeled for soybean aphid are efficacious. Some foliar insecticides have a 60-day preharvest interval, so check the label and the calendar when making product selections. At this time, product choice is not as critical as getting sufficient coverage," she says. "With any foliar application, strive for the highest knockdown possible to avoid resurgent aphid populations. Ideally, increasing volume and pressure will generate small droplets that should make contact with the aphids on the undersides of leaves. For ground applications, use 20 gallons of water per acre and 40 pounds of pressure per square inch." (See chart at right, courtesy ISU Extension).

Does spraying work? "Spraying at full seed set (R6) or later has not produced a consistent yield benefit in Iowa. In 2013, soybean aphid populations did not peak until late August and a yield response was not consistent in our efficacy evaluation," Hodgson adds. "Regardless of application timing, leave untreated check strips to assess if the treatment decision was profitable."

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